Among the previous processes that are suitable for the purification of paraxylene, crystallization is the one that has been used most, even though it is limited by a low recovery level due to the existence of a eutectic whose paraxylene concentration in the mixture reaches about 10 to 13%.
With the development of techniques for separation by adsorption in a molecular sieve, it is possible to achieve an excellent paraxylene yield, greater than, for example, 98%, independently of the eutectic composition limitation.
Usually, a recovery in excess of 98% of paraxylene is obtained with the processes of a simulated countercurrent fluid bed (U.S. Pat. No. 2,985,589) when the purity of the product is close to 95% by weight. A higher purity, exceeding 99%, however, can be reached at the expense of the yield.
Since an adsorption process makes it possible to carry out the separation of paraxylene with a high yield at the expense of purity while a crystallization process makes it possible to obtain a more pure product to the detriment of the yield, a hybrid process has been proposed that combines adsorption in a molecular sieve of aromatic C.sub.8 isomers, followed by crystallization of the paraxylene-enriched fraction (U.S. Pat. No. 5,401,476, and EP-B.531191 that are incorporated as references).
This process thus combines the advantages of a high yield and a very high purity of the wanted product compared to the processes of adsorption or of crystallization used separately.
Furthermore, the technology of crystallization is very old and has hardly been updated, considering the industrial breakthrough of the adsorption process in the simulated fluid bed.
The technological background is illustrated by the following documents: EP-A 611 589, EP-A 455 243, DE-A 2 222 755 and DE-C 972 036.
Crystallization for separating the paraxylene from a mixture of xylenes is generally carried out at very low temperatures, located in the range of those that can be attained by refrigeration with ethane or with ethylene. The costs of refrigeration and the consumption of energy are high, particularly because it is necessary to produce a cascade of cold cycles, with intermediate refrigeration with propane or with propene.
Certainly, this consumption is reduced when the operation is carried out in a higher range of crystallization temperatures, +10 to -30.degree. C., for example, as is the case when the crystallization feedstock is enriched to more than 50%, for example, preferably with more than 70% paraxylene, by an enrichment process by adsorption of xylenes or by paraselective dismutation of toluene, for example.
Moreover, the process of continuous crystallization by indirect exchange generally requires that the exchangers be scraped, which is an operation that is delicate and energy-intensive, regardless of the selected cooling level.